Thermodynamic investigation by heat capacity measurements of ferrimagnetic A2Mn[Mn(CN)6] (A=K, Rb, Cs) Prussian blue compounds

Abstract

Heat capacity measurements of a new series of Prussian blue analogs of A2Mn[Mn(CN)6] (A=K, Rb, Cs) composition were performed using thermal relaxation calorimetry. The Cs compound has a face-centered cubic structure with a linear Mn–C≡N–Mn linkage, while the monoclinic Rb and K compounds have nonlinear Mn–C≡N–Mn linkages. For all of the compounds, large broad thermal anomalies associated with magnetic transitions were observed in the temperature dependence of the heat capacity. The systematic changes in the heat capacity for the three compounds under magnetic fields of up to 7 T were found to be consistent with ferrimagnetic ordering with large spontaneous magnetization. Although the peak temperatures were slightly lower than reported values obtained by magnetic susceptibility measurements, the magnetic entropy was evaluated to be 22.0 ± 2.5 J K−1 mol−1. This value is consistent with an entropy of Rln12 corresponding to full entropy of one low-spin and one high-spin MnII ion in the formula unit, though some ambiguity remains in lattice estimation. Broadening of the peak width of the magnetic heat capacity divided by the temperature was observed as the size of the alkali ions decreased from Cs to K. This behavior is consistent with an increase in the lattice distortion produced by the bending of the C≡N–Mn angles.